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United States Patent |
6,099,965
|
Tennent
,   et al.
|
August 8, 2000
|
Rigid porous carbon structures, methods of making, methods of using and
products containing same
Abstract
This invention relates to rigid porous carbon structures and to methods of
making same. The rigid porous structures have a high surface area which
are substantially free of micropores. Methods for improving the rigidity
of the carbon structures include causing the nanofibers to form bonds or
become glued with other nanofibers at the fiber intersections. The bonding
can be induced by chemical modification of the surface of the nanofibers
to promote bonding, by adding "gluing" agents and/or by pyrolyzing the
nanofibers to cause fusion or bonding at the interconnect points.
Inventors:
|
Tennent; Howard (Kenneth Square, MA);
Moy; David (Winchester, MA);
Niu; Chun-Ming (Somerville, MA)
|
Assignee:
|
Hyperion Catalysis International, Inc. (Cambridge, MA)
|
Appl. No.:
|
857383 |
Filed:
|
May 15, 1997 |
Current U.S. Class: |
428/408; 264/29.1; 264/29.2; 264/29.3; 264/29.5; 264/29.6; 423/447.1; 423/447.2; 423/447.7; 423/450; 428/311.11; 428/312.2; 428/323; 428/367; 428/378 |
Intern'l Class: |
C01B 011/04 |
Field of Search: |
428/408,322,311.11,312.2,364,367,378
423/447.3,447.1,447.2,447.7,450
264/29.1,29.2,29.3,29.5,29.6
|
References Cited
U.S. Patent Documents
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|
4329260 | May., 1982 | Lester et al. | 252/446.
|
4518575 | May., 1985 | Porter et al. | 423/447.
|
4572813 | Feb., 1986 | Arakawa | 423/447.
|
4583299 | Apr., 1986 | Brooks | 502/182.
|
4642125 | Feb., 1987 | Burk et al. | 48/197.
|
4663230 | May., 1987 | Tennent | 428/408.
|
4816289 | Mar., 1989 | Komatsu et al. | 423/447.
|
4997804 | Mar., 1991 | Pekala | 502/418.
|
5081163 | Jan., 1992 | Pekala | 521/187.
|
5110693 | May., 1992 | Friend et al. | 429/40.
|
5165909 | Nov., 1992 | Tennent et al. | 428/408.
|
5238568 | Aug., 1993 | Fely et al. | 210/490.
|
5409683 | Apr., 1995 | Tillotson et al. | 423/338.
|
5439864 | Aug., 1995 | Rosin et al. | 502/180.
|
5456897 | Oct., 1995 | Moy et al. | 423/447.
|
5458784 | Oct., 1995 | Baker et al. | 210/674.
|
5494940 | Feb., 1996 | Unger et al. | 521/66.
|
5500200 | Mar., 1996 | Mandeville et al. | 423/447.
|
5569635 | Oct., 1996 | Moy et al. | 502/185.
|
5626650 | May., 1997 | Rodriguez et al. | 95/116.
|
5691054 | Nov., 1997 | Tennent et al. | 428/608.
|
5707916 | Jan., 1998 | Snyder et al. | 428/408.
|
5800706 | Sep., 1998 | Fischer | 210/198.
|
Primary Examiner: Turner; Archene
Attorney, Agent or Firm: Whitman Breed Abbott & Morgan LLP
Parent Case Text
This application claims priority from U.S. Provisional Appln. Ser. No.
60/020,804 filed May 15, 1996, hereby incorporated by reference.
Claims
We claim:
1. A rigid porous carbon structure which comprises intertwined,
interconnected carbon nanofibers, said rigid porous carbon structure
having a surface area greater than about 100 m.sup.2 /gm, being
substantially free of micropores and having a crush strength greater than
about 5 lb/in.sup.2.
2. The structure as recited in claim 1, wherein less than 1% of said
surface area is attributed to micropores.
3. The structure as recited in claim 1, wherein said structure has a carbon
purity greater than 95%.
4. The structure as recited in claim 1, wherein said structure has a
density greater than 0.8 g/cm.sup.3.
5. The structure as recited in claim 1, wherein said structure has a
density greater than 1.0 g/cm.sup.3.
6. The structure as recited in claim 1, wherein said structure has a
surface area greater than about 200 m.sup.2 /gm.
7. The structure as recited in claim 1, wherein said nanofibers are
uniformly and evenly distributed throughout said structure.
8. The structure as recited in claim 7, wherein the average distance
between nanofibers is less than about 0.03 microns and greater than about
0.005 microns.
9. The structure as recited in claim 7, wherein said structure comprises
substantially uniform pathways between said nanofibers.
10. The structure as recited in claim 1, wherein said nanofibers are in the
form of aggregate particles interconnected to form said structure.
11. The structure as recited in claim 10, wherein the average largest
distance between said individual aggregates is less than about 0.1 microns
and greater than about 0.001 microns.
12. The structure as recited in claim 10, wherein said structures comprise
aggregate spacings between the interconnected aggregate particles and
nanofiber spacings between said nanofibers within said aggregate
particles.
13. The structure as recited in claim 10, wherein said aggregate particles
are randomly entangled balls of nanofibers resembling bird nests.
14. The structure as recited in claim 10, wherein said aggregate particles
are bundles of nanofibers whose central axes are generally aligned
parallel to each other.
15. The structure as recited in claim 1, wherein said nanofibers have an
average diameter less than about 1 micron.
16. The structure as recited in claim 1, wherein said nanofibers are carbon
fibrils being substantially cylindrical with a substantially constant
diameter, having graphitic layers concentric with the fibril axis and
being substantially free of pyrolytically deposited carbon.
17. A rigid porous carbon structure which comprises intertwined,
interconnected carbon nanofibers, said rigid porous structure having a
surface area greater than about 100 m.sup.2 /gm, having a crush strength
greater than about 2 lb/in.sup.2, and a density greater than 0.8
g/cm.sup.3.
18. The structure as recited in claim 17, wherein said structure is
substantially free of micropores.
19. A method of preparing a rigid porous carbon structure having a surface
area greater than at least 100 m.sup.2 /gm, comprising the steps of:
(a) dispersing a plurality of nanofibers in a medium to form a suspension;
(b) separating said medium from said suspension to form said structure,
wherein said nanofibers are interconnected to form said rigid structure of
intertwined nanofibers bonded at the nanofibers intersections within the
structure.
20. The method as recited in claim 19, wherein said nanofibers are
uniformly and evenly distributed throughout said structure.
21. The method as recited in claim 19, wherein said carbon nanofibers are
in the form of aggregate particles interconnected to form said structure.
22. The method as recited in claim 19, wherein said aggregate particles are
evenly dispersed within said medium to form a slurry and said aggregate
particles are connected together with a gluing agent to form said
structure.
23. The method as recited in claim 19, wherein said medium is selected from
the group consisting of water and organic solvents.
24. The method as recited in claim 19, wherein said medium comprises a
dispersant selected from the group consisting of alcohols, glycerin,
surfactants, polyethylene glycol, polyethylene imines and polypropylene
glycol.
25. The method as recited in claim 19, wherein said nanofibers are surface
oxidized nanofibers that have been oxidized prior to dispersing in said
medium and said surface oxidized nanofibers are self-adhering forming a
said rigid structure by binding at the nanofiber intersections.
26. The method as recited in claim 25, wherein said structure is
subsequently pyrolized to remove oxygen.
27. The method as recited in claim 19, wherein said nanofibers are
dispersed in said suspension with gluing agents and said gluing agents
bond said nanofibers to form said rigid structure.
28. The method as recited in claim 27, wherein said gluing agent comprises
carbon.
29. The method as recited in claim 27, wherein said gluing agents are
selected from the group consisting of cellulose, carbohydrates,
polyethylene, polystyrene, nylon, polyurethane, polyester, polyamides and
phenolic resins.
30. The method as recited in claim 27, wherein said structure is
subsequently pyrolized to convert the gluing agent to carbon.
31. The method as recited in claim 19, wherein said step of separating
comprises filtering said suspension.
32. The method as recited in claim 19, wherein said step of separating
comprises evaporating said medium from said suspension.
33. The method as recited in claim 19, wherein said suspension is a gel or
paste comprising said nanofibers in a fluid and said separating comprises
the steps of:
(a) heating said gel or paste in a pressure vessel to a temperature above
the critical temperature of said fluid;
(b) removing supercritical fluid from said pressure vessel; and
(c) removing said structure from said pressure vessel.
34. A rigid porous carbon structure prepared by the method recited in claim
19.
35. A method of preparing a rigid porous carbon structure having a surface
area greater than at least 100 m.sup.2 /gm, comprising the steps of:
a) dispersing a plurality of nanofibers in a medium to form a suspension;
b) using a kneader to obtain a uniform, thick paste of the nanofiber
suspension;
c) extruding or pelletizing the paste;
d) separately said medium from said suspension to form said structure,
wherein said nanofibers are intertwined to form said rigid structure of
intertwined nanotubes bonded at the nanotube intersections within the
structure.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to rigid porous carbon structures. More
specifically, the invention relates to rigid three dimensional structures
comprising carbon nanofibers and having high surface area and porosity,
low bulk density, low amount of micropores and increased crush strength
and to methods of preparing and using such structures. The invention also
relates to using such rigid porous structures for a variety of purposes
including catalyst supports, electrodes, filters, insulators, adsorbents
and chromatographic media and to composite structures comprising the rigid
porous structures and a second material contained within the carbon
structures.
2. Description of the Related Art
Heterogeneous catalytic reactions are widely used in chemical processes in
the petroleum, petrochemical and chemical industries. Such reactions are
commonly performed with the reactant(s) and product(s) in the fluid phase
and the catalyst in the solid phase. In heterogeneous catalytic reactions,
the reaction occurs at the interface between phases, i.e., the interface
between the fluid phase of the reactant(s) and product(s) and the solid
phase of the supported catalyst. Hence, the properties of the surface of a
heterogeneous supported catalyst are significant factors in the effective
use of that catalyst. Specifically, the surface area of the active
catalyst, as supported, and the accessibility of that surface area to
reactant chemisorption and product desorption are important. These factors
affect the activity of the catalyst, i.e., the rate of conversion of
reactants to products. The chemical purity of the catalyst and the
catalyst support also have an important effect on the selectivity of the
catalyst, i.e., the degree to which the catalyst produces one product from
among several products, and the life of the catalyst.
Generally catalytic activity is proportional to catalyst surface area.
Therefore, high specific area is desirable. However, that surface area
must be accessible to reactants and products as well as to heat flow. The
chemisorption of a reactant by a catalyst surface is preceded by the
diffusion of that reactant through the internal structure of the catalyst
and the catalyst support, if any. The catalytic reaction of the reactant
to a product is followed by the diffusion of the product away from the
catalyst and catalyst support. Heat must be able to flow into and out of
the catalyst support as well.
Since the active catalyst compounds are often supported on the internal
structure of a support, the accessibility of the internal structure of a
support material to reactant(s), product(s) and heat flow is important.
Porosity and pore size distribution of the support structure are measures
of that accessibility. Activated carbons and charcoals used as catalyst
supports have surface areas of about 1000 square meters per gram and
porosities of less than one milliliter per gram. However, much of this
surface area and porosity, as much as 50%, and often more, is associated
with micropores, i.e., pores with pore diameters of 2 nanometers or less.
These pores can be difficult to access because of diffusion limitations.
Moreover, they are easily plugged and thereby deactivated. Thus, high
porosity materials where the pores are mainly in the mesopore (>2
nanometers) or macropore (>50 nanometers) ranges are most desirable.
It is also important that supported catalysts not fracture or attrit during
use because such fragments may become entrained in the reaction stream and
must then be separated from the reaction mixture. The cost of replacing
attritted catalyst, the cost of separating it from the reaction mixture
and the risk of contaminating the product are all burdens upon the
process. In other processes, e.g. where the solid supported catalyst is
filtered from the process stream and recycled to the reaction zone, the
fines may plug the filters and disrupt the process.
It is also important that a catalyst, at the very least, minimize its
contribution to the chemical contamination of reactant(s) and product(s).
In the case of a catalyst support, this is even more important since the
support is a potential source of contamination both to the catalyst it
supports and to the chemical process. Further, some catalysts are
particularly sensitive to contamination that can either promote unwanted
competing reactions, i.e., affect its selectivity, or render the catalyst
ineffective, i.e., "poison" it. Charcoal and commercial graphites or
carbons made from petroleum residues usually contain trace amounts of
sulfur or nitrogen as well as metals common to biological systems and may
be undesirable for that reason.
While activated charcoals and other carbon-containing materials have been
used as catalyst supports, none have heretofore had all of the requisite
qualities of porosity and pore size distribution, resistance to attrition
and purity for use in a variety of organic chemical reactions. For
example, as stated above, although these materials have high surface area,
much of the surface area is in the form of inaccessible micropores (i.e.,
diameter <2 nm).
Nanofiber mats, assemblages and aggregates have been previously produced to
take advantage of the high carbon purities and increased accessible
surface area per gram achieved using extremely thin diameter fibers. These
structures are typically composed of a plurality of intertwined or
intermeshed fibers. Although the surface area of these nanofibers is less
than an aerogel or activated large fiber, the nanofiber has a high
accessible surface area since the nanofibers are substantially free of
micropores.
One of the characteristics of the prior aggregates of nanofibers,
assemblages or mats made from nanofibers is low mechanical integrity and
high compressibility. Since the fibers are not very stiff these structures
are also easily compressed or deformed. As a result the size of the
structures cannot be easily controlled or maintained during use. In
addition, the nanofibers within the assemblages or aggregates are not held
together tightly. Accordingly, the assemblages and aggregates break apart
or attrit fairly easily. These prior mats, aggregates or assemblages are
either in the form of low porosity dense compressed masses of intertwined
fibers and/or are limited to microscopic structures.
Moreover, the above described compressibility of the nanofiber structures
may increase depending on a variety of factors including the method of
manufacture. For example, as suspensions of the nanofibers are drained of
a suspending fluid, in particular water, the surface tension of the liquid
tends to pull the fibrils into a dense packed "mat". The pore size of the
resulting mat is determined by the interfiber spaces which, because of the
compression of these mats, tend to be quite small. As a result, the fluid
flow characteristics of such mats are poor.
Alternatively, the structure may simply collapse under force or shear or
simply break apart. The above described nanofiber structures are typically
two fragile and/or too compressible to be used in such products as fixed
beds or chromatographic media. The force of the fluid flow causes the
flexible assemblages, mats or aggregates to compress, otherwise
restricting flow. The flow of a fluid through a capillary is described by
Poiseuille's equation which relates the flow rate to the pressure
differential, the fluid viscosity, the path length and size of the
capillaries. The rate of flow per unit area varies with the square of the
pore size. Accordingly, a pore twice as large results in flow rates four
times as large. The presence of pores of a substantially larger size in a
nanofiber structure results in increased fluid flow because the flow is
substantially greater through the larger pores. Decreasing the pore size
by compression dramatically reduces the flow. Moreover, such structures
also come apart when subjected to shear resulting in the individual
nanofibers breaking loose from the structure and be transported with the
flow.
As set forth above, prior aggregates, mats or assemblages provide
relatively low mechanical properties. Accordingly, although previous work
has shown that nanofibers can be assembled into thin, membrane-like or
particulate structures through which fluid will pass, such structures are
flexible and compressible and are subject to attrition. Accordingly, when
these structures are subjected to any force or shear, such as fluid or gas
flow, these structures collapse and/or compress resulting in a dense, low
porosity mass having reduced fluid flow characteristics. Moreover,
although the individual nanofibers have high internal surface areas, much
of the surface of the nanofiber structures is inaccessible due to the
compression of the structure and resulting decrease in pore size.
It would be desirable to produce a rigid porous carbon structure having
high accessible surface area, high porosity, increased rigidity and
significantly free from or no micropores. This is particularly true since
there are applications for porous carbon structures that require fluid
passage and/or high mechanical integrity. The compressibility and/or lack
of rigidity of previous structures of nanofibers creates serious
limitations or drawbacks for such applications. The mechanical and
structural characteristics of the rigid porous carbon structures brought
about by this invention make such applications more feasible and/or more
efficient.
OBJECTS OF THE INVENTION
It is therefore an object of this invention to provide rigid porous carbon
structures having high accessible surface area.
It is another object of the invention to provide a composition of matter
which comprises a three-dimensional rigid porous carbon structure
comprising carbon nanofibers.
It is a still further object to provide a rigid porous carbon structure
having non-carbon particulate matter or active sites dispersed within the
structure on the surface of the nanofibers.
It is yet another object of the invention to provide a composition of
matter comprising three-dimensional rigid porous carbon structure having a
low bulk density and high porosity to which can be added one or more
functional second materials in the nature of active catalysts,
electroactive species, etc. so as to form composites having novel
industrial properties.
It is a further object of the invention to provide processes for the
preparation of and methods of using the rigid porous carbon structures.
It is a still further object of the invention to provide improved catalyst
supports, filter media, chromatographic media, electrodes, EMI shielding
and other compositions of industrial value based on three-dimensional
rigid porous carbon structures.
It is a still further object of the invention to provide improved rigid
catalyst supports and supported catalysts for fixed bed catalytic
reactions for use in chemical processes in the petroleum, petrochemical
and chemical industries.
It is a still further object of the invention to provide improved,
substantially pure, rigid carbon catalyst support of high porosity,
activity, selectivity, purity and resistance to attrition.
It is a still further object of the invention to provide a rigid aerogel
composite comprising nanofibers.
It is a still further object of the invention to provide a rigid carbon
nanofiber mat comprising carbon particles on the mat surface.
The foregoing and other objects and advantages of the invention will be set
forth in or apparent from the following description and drawings.
SUMMARY OF THE INVENTION
The invention relates generally to rigid porous carbon structures and to
methods of making same. More specifically, it relates to rigid porous
structures having high surface area which are substantially free of
micropores. More particularly, the invention relates to increasing the
mechanical integrity and/or rigidity of porous structures comprising
intertwined carbon nanofibers.
The present invention provides methods for improving the rigidity of the
carbon structures by causing the nanofibers to form bonds or become glued
with other nanofibers at the fiber intersections. The bonding can be
induced by chemical modification of the surface of the nanofibers to
promote bonding, by adding "gluing" agents and/or by pyrolyzing the
nanofibers to cause fusion or bonding at the interconnect points.
The nanofibers within the porous structure can be in the form of discrete
fibers or aggregate particles of nanofibers. The former results in a
structure having fairly uniform properties. The latter results in a
structure having two-tiered architecture comprising an overall
macrostructure comprising aggregate particles of nanofibers bonded
together to form the porous mass and a microstructure of intertwined
nanofibers within the individual aggregate particles.
Another aspect of the invention relates to the ability to provide rigid
porous particulates of a specified size dimension, for example, porous
particulates of a size suitable for use in a fluidized packed bed. The
method involves preparing a plurality of carbon nanofibers or aggregates,
fusing the nanofibers at their intersections or aggregates to form a large
bulk solid mass and sizing the solid mass down into pieces of rigid porous
high surface area particulates having a size suitable for the desired use,
for example, to a particle size suitable for forming a packed bed.
According to another aspect of the invention, the nanofibers are
incorporated in an aerogel or xerogel composite through sol-gel
polymerization.
According to another embodiment of the invention, the structures are used
as filter media, as catalyst supports, filters, adsorbents, as
electroactive materials for use, e.g. in electrodes in fuel cells and
batteries, and as chromatography media. It has been found that the carbon
structures are useful in the formation of composites which comprise the
structure together with either a particulate solid, an electroactive
component or a catalytically active metal or metal-containing compound.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a SEM photomicrograph of a rigid porous carbon structure made
from oxidized fibrils followed by pyrolysis.
FIG. 2 is a graphical representation of the cumulative pore volume by
absorption/desorption for a rigid carbon structure made from oxidized
nanofibers wherein the vertical axis represents desorption pore volume and
the horizontal axis represents pore diameter.
FIG. 3 is a graphical representation of a cumulative pore volume by
absorption/desorption for a rigid carbon structure made from "as is"
nanofibers wherein the vertical axis represents desorption pore volume and
the horizontal axis represents pore diameter.
FIG. 4 is a flow diagram of methods of making composite xerogels and
aerogels according to one embodiment of the invention.
DEFINITIONS
The term "assemblage", "mat" or "aggregate" refers to any configuration of
a mass of intertwined individual nanofibers. The term "assemblage"
includes open loose structures having uniform properties. The term "mat"
refers to a relatively dense felt-like structure. The term "aggregate"
refers to a dense, microscopic particulate structure. More specifically,
the term "assemblage" refers to structures having relatively or
substantially uniform physical properties along at least one dimensional
axis and desirably have relatively or substantially uniform physical
properties in one or more planes within the assemblage, i.e. they have
isotropic physical properties in that plane. The assemblage may comprise
uniformly dispersed individual interconnected nanofibers or a mass of
connected aggregates of nanofibers. In other embodiments, the entire
assemblage is relatively or substantially isotropic with respect to one or
more of its physical properties. The physical properties which can be
easily measured and by which uniformity or isotropy are determined include
resistivity and optical density.
The term "accessible surface area" refers to that surface area not
attributed to micropores (i.e., pores having diameters or cross-sections
less than 2 nm).
The term "fluid flow rate characteristic" refers to the ability of a fluid
or gas to pass through a solid structure. For example, the rate at which a
volume of a fluid or gas passes through a three-dimensional structure
having a specific cross-sectional area and specific thickness or height
differential across the structure (i.e. milliliters per minute per square
centimeter per mil thickness).
The term "isotropic" means that all measurements of a physical property
within a plane or volume of the structure, independent of the direction of
the measurement, are of a constant value. It is understood that
measurements of such non-solid compositions must be taken on a
representative sample of the structure so that the average value of the
void spaces is taken into account.
The term "nanofiber" refers to elongated structures having a cross section
(e.g., angular fibers having edges) or diameter (e.g., rounded) less than
1 micron. The structure may be either hollow or solid. Accordingly, the
term includes "bucky tubes" and "nanotubes". This term is defined further
below.
The term "internal structure" refers to the internal structure of an
assemblage including the relative orientation of the fibers, the diversity
of and overall average of fiber orientations, the proximity of the fibers
to one another, the void space or pores created by the interstices and
spaces between the fibers and size, shape, number and orientation of the
flow channels or paths formed by the connection of the void spaces and/or
pores. According to another embodiment, the structure may also include
characteristics relating to the size, spacing and orientation of aggregate
particles that form the assemblage. The term "relative orientation" refers
to the orientation of an individual fiber or aggregate with respect to the
others (i.e., aligned versus non-aligned). The "diversity of" and "overall
average" of fiber or aggregate orientations refers to the range of fiber
orientations within the structure (alignment and orientation with respect
to the external surface of the structure).
The term "physical property" means an inherent, measurable property of the
porous structure, e.g., surface area, resistivity, fluid flow
characteristics, density, porosity, etc.
The term "relatively" means that ninety-five percent of the values of the
physical property when measured along an axis of, or within a plane of or
within a volume of the structure, as the case may be, will be within plus
or minus 20 percent of a mean value.
The term "substantially" means that ninety-five percent of the values of
the physical property when measured along an axis of, or within a plane of
or within a volume of the structure, as the case may be, will be within
plus or minus ten percent of a mean value.
The terms "substantially isotropic" or "relatively isotropic" correspond to
the ranges of variability in the values of a physical property set forth
above.
DETAILED DESCRIPTION OF THE INVENTION
Nanofibers
The term nanofibers refers to various fibers, particularly carbon fibers,
having very small diameters including fibrils, whiskers, nanotubes,
buckytubes, etc. Such structures provide significant surface area when
incorporated into a structure because of their size and shape. Moreover,
such fibers can be made with high purity and uniformity.
Preferably, the nanofiber used in the present invention has a diameter less
than 1 micron, preferably less than about 0.5 micron, and even more
preferably less than 0.1 micron and most preferably less than 0.05 micron.
According to one preferred embodiment of the invention, carbon fibrils are
used to form the rigid assemblage. Carbon fibrils can be made having
diameters in the range of 3.5 to 70 nanometers.
The fibrils, buckytubes, nanotubes and whiskers that are referred to in
this application are distinguishable from continuous carbon fibers
commercially available as reinforcement materials. In contrast to
nanofibers, which have desirably large, but unavoidably finite aspect
ratios, continuous carbon fibers have aspect ratios (L/D) of at least
10.sup.4 and often 10.sup.6 or more. The diameter of continuous fibers is
also far larger than that of fibrils, being always >1.0 .mu.m and
typically 5 to 7 .mu.m.
Continuous carbon fibers are made by the pyrolysis of organic precursor
fibers, usually rayon, polyacrylonitrile (PAN) and pitch. Thus, they may
include heteroatoms within their structure. The graphitic nature of "as
made" continuous carbon fibers varies, but they may be subjected to a
subsequent graphitization step. Differences in degree of graphitization,
orientation and crystallinity of graphite planes, if they are present, the
potential presence of heteroatoms and even the absolute difference in
substrate diameter make experience with continuous fibers poor predictors
of nanofiber chemistry.
Carbon fibrils are vermicular carbon deposits having diameters less than
1.0.mu., preferably less than 0.5.mu., even more preferably less than
0.2.mu. and most preferably less than 0.05.mu.. They exist in a variety of
forms and have been prepared through the catalytic decomposition of
various carbon-containing gases at metal surfaces. Such vermicular carbon
deposits have been observed almost since the advent of electron
microscopy. A good early survey and reference is found in Baker and
Harris, Chemistry and Physics of Carbon, Walker and Thrower ed., Vol. 14,
1978, p. 83 and Rodriguez, N., J. Mater. Research, Vol. 8, p. 3233 (1993),
each of which are hereby incorporated by reference. (see also, Obelin, A.
and Endo, M., J. of Crystal Growth, Vol. 32 (1976), pp. 335-349, hereby
incorporated by reference).
U.S. Pat. No. 4,663,230 to Tennent, hereby incorporated by reference,
describes carbon fibrils that are free of a continuous thermal carbon
overcoat and have multiple ordered graphitic outer layers that are
substantially parallel to the fibril axis. As such they may be
characterized as having their c-axes, the axes which are perpendicular to
the tangents of the curved layers of graphite, substantially perpendicular
to their cylindrical axes. They generally have diameters no greater than
0.1.mu. and length to diameter ratios of at least 5. Desirably they are
substantially free of a continuous thermal carbon overcoat, i.e.,
pyrolytically deposited carbon resulting from thermal cracking of the gas
feed used to prepare them. The Tennent invention provided access to
smaller diameter fibrils, typically 35 to 700 .ANG. (0.0035 to 0.070 .mu.)
and to an ordered, "as grown" graphitic surface. Fibrillar carbons of less
perfect structure, but also without a pyrolytic carbon outer layer have
also been grown.
U.S. Pat. No. 5,171,560 to Tennent et al., hereby incorporated by
reference, describes carbon fibrils free of thermal overcoat and having
graphitic layers substantially parallel to the fibril axes such that the
projection of said layers on said fibril axes extends for a distance of at
least two fibril diameters. Typically, such fibrils are substantially
cylindrical, graphitic nanotubes of substantially constant diameter and
comprise cylindrical graphitic sheets whose c-axes are substantially
perpendicular to their cylindrical axis. They are substantially free of
pyrolytically deposited carbon, have a diameter less than 0.1.mu. and a
length to diameter ratio of greater than 5. These fibrils are of primary
interest in the invention.
When the projection of the graphitic layers on the fibril axis extends for
a distance of less than two fibril diameters, the carbon planes of the
graphitic nanofiber, in cross section, take on a herring bone appearance.
These are termed fishbone fibrils. Geus, U.S. Pat. No. 4,855,091, hereby
incorporated by reference, provides a procedure for preparation of
fishbone fibrils substantially free of a pyrolytic overcoat. These fibrils
are also useful in the practice of the invention.
According to one embodiment of the invention, oxidized nanofibers are used
to form the rigid porous assemblage. McCarthy et al., U.S. patent
application Ser. No. 351,967 filed May 15, 1989, hereby incorporated by
reference, describes processes for oxidizing the surface of carbon fibrils
that include contacting the fibrils with an oxidizing agent that includes
sulfuric acid (H.sub.2 SO.sub.4) and potassium chlorate (KClO.sub.3) under
reaction conditions (e.g., time, temperature, and pressure) sufficient to
oxidize the surface of the fibril. The fibrils oxidized according to the
processes of McCarthy, et al. are non-uniformly oxidized, that is, the
carbon atoms are substituted with a mixture of carboxyl, aldehyde, ketone,
phenolic and other carbonyl groups.
Fibrils have also been oxidized non-uniformly by treatment with nitric
acid. International Application PCT/US94/10168 discloses the formation of
oxidized fibrils containing a mixture of functional groups. Hoogenraad, M.
S., et al. ("Metal Catalysts supported on a Novel Carbon Support",
Presented at Sixth International Conference on Scientific Basis for the
Preparation of Heterogeneous Catalysts, Brussels, Belgium, September,
1994) also found it beneficial in the preparation of fibril-supported
precious metals to first oxidize the fibril surface with nitric acid. Such
pretreatment with acid is a standard step in the preparation of
carbon-supported noble metal catalysts, where, given the usual sources of
such carbon, it serves as much to clean the surface of undesirable
materials as to functionalize it.
In published work, McCarthy and Bening (Polymer Preprints ACS Div. of
Polymer Chem. 30 (1)420(1990)) prepared derivatives of oxidized fibrils in
order to demonstrate that the surface comprised a variety of oxidized
groups. The compounds they prepared, phenylhydrazones, haloaromaticesters,
thallous salts, etc., were selected because of their analytical utility,
being, for example, brightly colored, or exhibiting some other strong and
easily identified and differentiated signal. These compounds were not
isolated and are, unlike the derivatives described herein, of no practical
significance.
The nanofibers may be oxidized using hydrogen peroxide, chlorate, nitric
acid and other suitable reagents.
The nanofibers within the structure may be further functionalized as set
forth in U.S. patent application Ser. No. 08/352,400, filed Dec. 8, 1995,
by Hoch and Moy et al., entitled "Functionalized Fibrils", hereby
incorporated by reference.
Carbon nanotubes of a morphology similar to the catalytically grown fibrils
described above have been grown in a high temperature carbon arc (Iijima,
Nature 354 56 1991, hereby incorporated by reference). It is now generally
accepted (Weaver, Science 265 1994, hereby incorporated by reference) that
these arc-grown nanofibers have the same morphology as the earlier
catalytically grown fibrils of Tennent. Arc grown carbon nanofibers are
also useful in the invention.
The nanofibers may also be high surface area nanofibers disclosed in U.S.
Provisional Application Ser. No. 60/017,787 (CMS Docket No.: 370077-3630)
entitled "High Surface Area Nanofibers, Methods of Making, Methods of
Using and Products Containing Same", filed concurrently, hereby
incorporated by reference.
Nanofiber Aggregates and Assemblages
The "unbonded" precursor nanofibers may be in the form of discrete fibers,
aggregates of fibers or both.
When carbon fibrils are used, the aggregates, when present, are generally
of the bird's nest, combed yarn or open net morphologies. The more
"entangled" the aggregates are, the more processing will be required to
achieve a suitable composition if a high porosity is desired. This means
that the selection of combed yarn or open net aggregates is most
preferable for the majority of applications. However, bird's nest
aggregates will generally suffice.
The nanofiber mats or assemblages have been prepared by dispersing
nanofibers in aqueous or organic mediums and then filtering the nanofibers
to form a mat or assemblage. Assemblages have also been prepared by
intimately mixing nanofibers with carbonizable resins, such as phenolic
resins, in a kneader, followed by extruding or pelletizing and pyrolizing.
The mats have also been prepared by forming a gel or paste of nanofibers
in a fluid, e.g. an organic solvent such as propane and then heating the
gel or paste to a temperature above the critical temperature of the
medium, removing supercritical fluid and finally removing the resultant
porous mat or plug from the vessel in which the process has been carried
out. See, parent U.S. patent application Ser. No. 08/428,496 entitled
"Three-Dimensional Macroscopic Assemblages of Randomly Oriented Carbon
Fibrils and Composites Containing Same" by Tennent et al., hereby
incorporated by reference.
Nanofibers may also be prepared as aggregates having various morphologies
(as determined by scanning electron microscopy) in which they are randomly
entangled with each other to form entangled balls of nanofibers resembling
bird nests ("BN"); or as aggregates consisting of bundles of straight to
slightly bent or kinked carbon nanofibers having substantially the same
relative orientation, and having the appearance of combed yarn ("CY")
e.g., the longitudinal axis of each nanofiber (despite individual bends or
kinks) extends in the same direction as that of the surrounding nanofibers
in the bundles; or, as, aggregates consisting of straight to slightly bent
or kinked nanofibers which are loosely entangled with each other to form
an "open net" ("ON") structure. In open net structures the of nanofiber
entanglement is greater than observed in the combed yarn aggregates (in
which the individual nanofibers have substantially the same relative
orientation) but less than that of bird nest. CY and ON aggregates are
more readily dispersed than BN making them useful in composite fabrication
where uniform properties throughout the structure are desired. The
substantial linearity of the individual nanofiber strands also makes the
aggregates useful in EMI shielding and electrical applications.
The morphology of the aggregate is controlled by the choice of catalyst
support. Spherical supports grow nanofibers in all directions leading to
the formation of bird nest aggregates. Combed yarn and open nest
aggregates are prepared using supports having one or more readily
cleavable planar surfaces, e.g., an iron or iron-containing metal catalyst
particle deposited on a support material having one or more readily
cleavable surfaces and a surface area of at least 1 square meters per
gram. Moy et al., U.S. application Ser. No. 08/469,430 entitled "Improved
Methods and Catalysts for the Manufacture of Carbon Fibrils", filed Jun.
6, 1995, hereby incorporated by reference, describes fibrils prepared as
aggregates having various morphologies (as determined by scanning electron
microscopy).
Further details regarding the formation of carbon nanofiber aggregates may
be found in the disclosure of U.S. Pat. No. 5,165,909 to Tennent; U.S.
Pat. No. 5,456,897 to Moy et al.; Snyder et al., U.S. patent application
Ser. No. 149,573, filed Jan. 28, 1988, and PCT Application No. US89/00322,
filed Jan. 28, 1989 ("Carbon Fibrils") WO 89/07163, and Moy et al., U.S.
patent application Ser. No. 413,837 filed Sep. 28, 1989 and PCT
Application No. US90/05498, filed Sep. 27, 1990 ("Fibril Aggregates and
Method of Making Same") WO 91/05089, and U.S. application Ser. No.
08/479,864 to Mandeville et al., filed Jun. 7, 1995 and U.S. application
Ser. No. 08/329,774 by Bening et al., filed Oct. 27, 1984 and U.S.
application Ser. No. 08/284,917, filed Aug. 2, 1994 and U.S. application
Ser. No. 07/320,564, filed Oct. 11, 1994 by Moy et al., all of which are
assigned to the same assignee as the invention here and are hereby
incorporated by reference.
Hard, Porous Carbon Structures and Methods of Preparing Same
The invention relates to methods for producing rigid, porous structures
from nanofibers. The resulting structures may be used in catalysis,
chromatography, filtration systems, electrodes and batteries, etc.
1. Rigid Porous Carbon Nanofiber Structures
The rigid porous carbon structures according to the invention have high
accessible surface area. That is, the structures have a high surface area,
but are substantially free of micropores (i.e., pores having a diameter or
cross-section less than 2 nm). The invention relates to increasing the
mechanical integrity and/or rigidity of porous structures comprising
intertwined carbon nanofibers. The structures made according to the
invention have higher crush strengths than the conventional nanofiber
structures. The present invention provides a method of improving the
rigidity of the carbon structures by causing the nanofibers to form bonds
or become glued with other nanofibers at the fiber intersections. The
bonding can be induced by chemical modification of the surface of the
nanofibers to promote bonding, by adding "gluing" agents and/or by
pyrolyzing the nanofibers to cause fusion or bonding at the interconnect
points.
The nanofibers can be in the form of discrete fibers or aggregate particles
of nanofibers. The former results in a structure having fairly uniform
properties. The latter results in a structure having two-tiered
architecture comprising an overall macrostructure comprising aggregate
particles of nanofibers bonded together and a microstructure of
intertwined nanofibers within the individual aggregate particles.
According to one embodiment, individual discrete nanofibers form the
structure. In these cases, the distribution of individual fibril strands
in the particles are substantially uniform with substantially regular
spacing between strands. These spacings (analogous to pores in
conventional supports) varied according to the densities of the structures
and ranged roughly from 15 nm in the densest (pressed disc from oxidized
fibrils, density=1-1.2 g/cc) to an average 50-60 nm in the lightest
particles (e.g., solid mass formed from open net aggregates). Absent are
cavities or spaces that would correspond to micropores (<2 nm) in
conventional carbon supports. FIG. 1 is a SEM photograph of a nanofiber
structure formed using individual oxidized nanofibers.
These rigid porous materials are superior to currently available high
surface area materials for use in fixed-bed carbon-supported catalysts,
for example. The ruggedness of the structures, the porosity (both pore
volume and pore structure), and the purity of the carbon are significantly
improved. Combining these properties with relatively high surface areas
provides a unique material with useful characteristics. Additionally, no
other carbon support (perhaps no other of any kind) has surface areas as
high as 400 m.sup.2 /g without having much of the area buried in
inaccessible micropores.
One embodiment of the invention relates to a rigid porous carbon structure
having an accessible surface area greater than about 100 m.sup.2 /gm,
being substantially free of micropores and having a crush strength greater
than about 1 lb. Preferably, the structure comprises intertwined,
interconnected carbon nanofibers wherein less than 1% of said surface area
is attributed to micropores.
Preferably, the structures have a carbon purity greater than 50 wt %, more
preferably greater than 80 wt %, even more preferably greater than 95 wt %
and most preferably greater than 99 wt %.
Preferably, the structures made from oxidized nanofibers (measured in the
form of 1/8 inch diameter cylindrical extrudates) have a crush strength
greater than 5 lb/in.sup.2, more preferably greater than 10 lb/in.sup.2,
even more preferably greater than 15 lb/in.sup.2 and most preferably
greater than 20 lb/in.sup.2.
Preferably, the structures made from "as is" nanofibers (measured in the
form of 1/4 diameter pellets) have a crush strength greater than 20
lb/in.sup.2, more preferably greater than about 40 lb/in.sup.2, even more
preferably greater than about 60 lb/in.sup.2 and most preferably greater
than about 70 lb/in.sup.2.
According to another embodiment, the rigid porous carbon structure having
an accessible surface area greater than about 100 m.sup.2 /gm, having a
crush strength greater than about 5 lb/in.sup.2, and a density greater
than 0.8 g/cm.sup.3. Preferably, the structure is substantially free of
micropores.
According to yet another embodiment, the rigid porous carbon structure has
an accessible surface area greater than about 100 m.sup.2 /gm, porosity
greater than 0.5 cc/g, being substantially free of micropores, having a
carbon purity greater than 95 wt % and having a crush strength greater
than about 5 lb/in.sup.2.
The structure preferably has a density greater than 0.8 g/cm.sup.3.
According to another embodiment, the structure preferably has a density
greater than 1.0 g/cm.sup.3.
Preferably, the structure has an accessible surface area greater than about
100 m.sup.2 /g, more preferably greater than 150 m.sup.2 /g, even more
preferably greater than 200 m.sup.2 /g, even more preferably greater than
300 m.sup.2 /g, and most preferably greater than 400 m.sup.2 /g.
According to one embodiment, the structure comprises nanofibers which are
uniformly and evenly distributed throughout said structure. That is, the
structure is a rigid and uniform assemblage of nanofibers. The structure
comprises substantially uniform pathways and spacings between said
nanofibers. The pathways or spacings are uniform in that each has
substantially the same cross-section and are substantially evenly spaced.
Preferably, the average distance between nanofibers is less than about
0.03 microns and greater than about 0.005 microns. The average distance
may vary depending on the density of the structure.
According to another embodiment, the structure comprises nanofibers in the
form of nanofiber aggregate particles interconnected to form said
structure. These structures comprise larger aggregate spacings between the
interconnected aggregate particles and smaller nanofiber spacings between
the individual nanofibers within the aggregate particles. Preferably, the
average largest distance between said individual aggregates is less than
about 0.1 microns and greater than about 0.001 microns. The aggregate
particles may include, for example, particles of randomly entangled balls
of nanofibers resembling bird nests and/or bundles of nanofibers whose
central axes are generally aligned parallel to each other.
The nanofibers have an average diameter less than about 1 micron.
Preferably, less than 0.5 microns, more preferably less than 0.1 micron,
even more preferably less than 0.05 microns, and most preferably less than
0.01 microns.
Preferably, the nanofibers are carbon fibrils being substantially
cylindrical with a substantially constant diameter, having graphitic or
graphenic layers concentric with the fibril axis and being substantially
free of pyrolytically deposited carbon.
Another aspect of the invention relates to the ability to provide rigid
porous particulates or pellets of a specified size dimension. For example,
porous particulates or pellets of a size suitable for use in a fluidized
packed bed. The method involves preparing a plurality of carbon nanofibers
or aggregates, fusing or gluing the aggregates or nanofibers at their
intersections to form a large rigid bulk solid mass and sizing the solid
mass down into pieces of rigid porous high surface area particulates
having a size suitable for the desired use, for example, to a particle
size suitable for forming a packed bed.
The above-described structures are formed by causing the nanofibers to form
bonds or become glued with other nanofibers at the fiber intersections.
The bonding can be induced by chemical modification of the surface of the
nanofibers to promote bonding, by adding "gluing" agents and/or by
pyrolyzing the nanofibers to cause fusion or bonding at the interconnect
points.
The hard, high porosity structures can be formed from regular nanofibers or
nanofiber aggregates, either with or without surface modified nanofibers
(i.e., surface oxidized nanofibers). In order to increase the stability of
the nanofiber structures, it is also possible to deposit polymer at the
intersections of the structure. This may be achieved by infiltrating the
assemblage with a dilute solution of low molecular weight polymer cement
(i.e., less than about 1,000 MW) and allowing the solvent to evaporate.
Capillary forces will concentrate the polymer at nanofiber intersections.
It is understood that in order to substantially improve the stiffness and
integrity of the structure, only a small fraction of the nanofiber
intersections need be cemented.
One embodiment of the invention relates to a method of preparing a rigid
porous carbon structure having a surface area greater than at least 100
m.sup.2 /gm, comprising the steps of:
(a) dispersing a plurality of nanofibers in a medium to form a suspension;
and
(b) separating said medium from said suspension to form said structure,
wherein said nanofibers are interconnected to form said rigid structure of
intertwined nanotubes bonded at nanofiber intersections within the
structure.
The nanofibers may be uniformly and evenly distributed throughout the
structure or in the form of aggregate particles interconnected to form the
structure. When the former is desired, the nanofibers are dispersed
thoroughly in the medium to form a dispersion of individual nanofibers.
When the latter is desired, nanofiber aggregates are dispersed in the
medium to form a slurry and said aggregate particles are connected
together with a gluing agent to form said structure.
The medium used may be selected from the group consisting of water and
organic solvents. Preferably, the medium comprises a dispersant selected
from the group consisting of alcohols, glycerin, surfactants, polyethylene
glycol, polyethylene imines and polypropylene glycol.
The medium should be selected which: (1) allows for fine dispersion of the
gluing agent in the aggregates; and (2) also acts as a templating agent to
keep the internal structure of the aggregates from collapsing as the mix
dries down.
One preferred embodiment employs a combination of polyethylene glycol (PEG)
and glycerol dissolved in water or alcohol as the dispersing medium, and a
carbonizable material such as low MW phenol-formaldehyde resins or other
carbonizable polymers or carbohydrates (starch or sugar).
If surface oxidized nanofibers are employed, the nanofibers are oxidized
prior to dispersing in the medium and are self-adhering forming the rigid
structure by binding at the nanofiber intersections. The structure may be
subsequently pyrolized to remove oxygen.
According to another embodiment, the nanofibers are dispersed in said
suspension with gluing agents and the gluing agents bond said nanofibers
to form said rigid structure. Preferably, the gluing agent comprises
carbon, even more preferably the gluing agent is selected from a material
that, when pyrolized, leaves only carbon. Accordingly, the structure
formed with such a gluing may be subsequently pyrolized to convert the
gluing agent to carbon.
Preferably, the gluing agents are selected from the group consisting of
cellulose, carbohydrates, polyethylene, polystyrene, nylon, polyurethane,
polyester, polyamides and phenolic resins.
According to further embodiments of the invention, the step of separating
comprises filtering the suspension or evaporating the medium from said
suspension.
According to yet another embodiment, the suspension is a gel or paste
comprising the nanofibers in a fluid and the separating comprises the
steps of:
(a) heating the gel or paste in a pressure vessel to a temperature above
the critical temperature of the fluid;
(b) removing supercritical fluid from the pressure vessel; and
(c) removing the structure from the pressure vessel.
Isotropic slurry dispersions of nanofiber aggregates in solvent/dispersant
mixtures containing gluing agent can be accomplished using a Waring
blender or a kneader without disrupting the aggregates. The nanofiber
aggregates trap the resin particles and keep them distributed.
These mixtures can be used as is, or can be filtered to remove sufficient
solvent to obtain cakes with high nanofiber contents (.about.5-20% dry
weight basis). The cake can be molded, extruded or pelletized. The molded
shapes are sufficiently stable so that further drying occurs without
collapse of the form. On removing solvent, disperant molecules, along with
particles of gluing agent are concentrated and will collect at nanofiber
crossing points both within the nanofiber aggregates, and at the outer
edges of the aggregates. As the mixture is further dried down and
eventually carbonized, nanofiber strands within the aggregates and the
aggregates themselves are glued together at contact points. Since the
aggregate structures do not collapse, a relatively hard, very porous, low
density particle is formed.
As set forth above, the rigid, porous structures may also be formed using
oxidized nanofibers with or without a gluing agent. Carbon nanofibers
become self-adhering after oxidation. Very hard, dense mats are formed by
highly dispersing the oxidized nanofibers (as individualized strands),
filtering and drying. The dried mats have densities between 1-1.2 g/cc,
depending on oxygen content, and are hard enough to be ground and sized by
sieving. Measured surface areas are about 275 m.sup.2 /g.
Substantially all the oxygen within the resulting rigid structure can be
removed by pyrolizing the particles at about 600.degree. C. in flowing
gas, for example argon. Densities decrease to about 0.7-0.9 g/cc and the
surface areas increase to about 400 m.sup.2/ g. Pore volumes for the
calcined particles are about 0.9-0.6 cc/g, measured by water absorbtion.
The oxidized nanofibers may also be used in conjunction with a gluing
agent. Oxidized nanofibers are good starting materials since they have
attachment points to stick both gluing agents and templating agents. The
latter serve to retain the internal structure of the particles or mats as
they dry, thus preserving the high porosity and low density of the
original nanofiber aggregates. Good dispersions are obtained by slurrying
oxidized nanofibers with materials such as polyethyleneimine cellulose
(PEI Cell), where the basic imine functions form strong electrostatic
interactions with carboxylic acid functionalized fibrils. The mix is
filtered to form mats. Pyrolizing the mats at temperatures greater than
650.degree. C. in an inert atmosphere converts the PEI Cell to carbon
which acts to fuse the nanofiber aggregates together into hard structures.
The result is a rigid, substantially pure carbon structure.
Solid ingredients can also be incorporated within the structure by mixing
the additives with the nanofiber dispersion prior to formation of the
structure. The content of other solids in the dry structure may be made as
high as fifty parts solids per part of nanofibers.
According to one preferred embodiment, nanofibers are dispersed at high
shear in a high-shear mixer, e.g. a Waring Blender. The dispersion may
contain broadly from 0.01 to 10% nanofibers in water, ethanol, mineral
spirits, etc. This procedure adequately opens nanofiber bundles, i.e.
tightly wound bundles of nanofibers, and disperses the nanofibers to form
self-supporting mats after filtration and drying. The application of high
shear mixing may take up to several hours. Mats prepared by this method,
however, are not free of aggregates.
If the high shear procedure is followed by ultrasonication, dispersion is
improved. Dilution to 0.1% or less aids ultrasonication. Thus, 200 cc of
0.1% fibrils, for example, may be sonified by a Bronson Sonifier Probe
(450 watt power supply) for 5 minutes or more to further improve the
dispersion.
To achieve the highest degrees of dispersion, i.e. a dispersion which is
free or virtually free of fibril aggregates, sonication must take place
either at very low concentration in a compatible liquid, e.g. at 0.001% to
0.01% concentration in ethanol or at higher concentration e.g. 0.1% in
water to which a surfactant, e.g. Triton X-100, has been added in a
concentration of about 0.5%. The mat which is subsequently formed may be
rinsed free or substantially free of surfactant by sequential additions of
water followed by vacuum filtration.
Particulate solids such as MnO.sub.2 (for batteries) and Al.sub.2 O.sub.3
(for high temperature gaskets) may be added to the nanofiber dispersion
prior to mat formation at up to 50 parts added solids per part of fibrils.
Reinforcing webs and scrims may be incorporated on or in the mats during
formation. Examples are polypropylene mesh and expanded nickel screen.
Lightly oxidized (i.e., with 30% H.sub.2 O.sub.2) nanofiber aggregates
still disperse as aggregates, rather than as individualized nanofibers.
Bonding these structures together retains the high porosities and low
densities of the original nanofibers.
According to one embodiment, discs (1/2 inch in diam) were prepared by
isostatic pressing the dried powders of oxidized nanofibers. Densities of
the discs, which are related to oxygen content, could be varied by thermal
treatment of the discs. Hard particles with high densities and
intermediate porosities can be formed by these methods. Rigid, porous
structures made from BN and CC production nanofiber aggregates with and
without any prior chemical treatment have been made using phenolic resins
or other organic polymers as gluing agents, and their properties are
summarized in the Table I.
TABLE I
______________________________________
Summary of Physical Properties of Formed Structures.
Fibril or Aggregate Water
Type Density g/cc
Absorp. cc/g
______________________________________
Oxid. Mats, uncalc.
1-1.2 0.6-0.3
Oxidized Mats, calc.
0.7-.9 0.6-0.9
BN (Green Disc)
1.74 --
BN (600.degree. C. Disc)
1.59 --
BN (900.degree. C. Disc)
1.56 --
CC (Green Disc)
1.33 --
CC (600.degree. C. Disc)
1.02 0.6
CC (900.degree. C. Disc)
0.95 0.6
PU-BN (20%) 0.7 0.9
PS-CC (15%) 0.6 1.1
PE-BN (20%) 0.4 3.5
CC (2) 0.15 6.0
BN (2) 0.30 2.8
CC (2) 0.14 6.5
BN (2) 0.31 2.6
CC (2) 0.27 3.2
BN (2) 0.50 1.5
CC (2) 0.23 3.8
CC (2) 0.32 2.6
CC (3) 0.33 2.5
CC (3) 0.47 1.7
______________________________________
(1) Oxidized Fibrils
(2) Asgrown Fibrils/Dispersant/Gluing Agent
(3) Asgrown Fibrils/PEG/Bakelite ResinExtrudates
The structures may also be useful in capacitors as set forth in U.S.
Provisional Application Ser. No. 60/017,609 (CMS Docket No.: 370077-3600)
entitled "GRAPHITIC NANOFIBERS IN ELECTROCHEMICAL CAPACITORS", filed
concurrently, hereby incorporated by reference.
Another aspect of the invention relates to the formation of aerogel or
xerogel composites comprising nanofibers to form a rigid porous structure.
Aerogels are a unique class of materials with extremely low density, high
porosity and surface areas. Organic aerogels and carbon aerogels, as
exemplified by R. W. Pekala's publications, are attractive for many
applications including high density energy storage, high capacity
absorbents and catalysts supports. Similar materials, so called foamed
organic polymer with relatively low density are well known and are widely
used as insulating materials. Conventional monolithic organic aerogels
have very poor mechanical properties. In most cases, the aerogels are
insulators. Therefore, it is of interest to prepare aerogel composites
with improved mechanical and electronic properties. An xerogel is similar
to an aerogel, but has a denser structure as a result of the method of
manufacture (see FIG. 4).
Such structures are set forth more fully in U.S. Pat. Nos. 5,476,878 to
Pekala; U.S. Pat. No. 5,124,100 to Nishii et al.; U.S. Pat. No. 5,494,940
to Unger et al.; U.S. Pat. No. 5,416,376 to Wuest et al; U.S. Pat. No.
5,409,683 to Tillotson et al.; U.S. Pat. No. 5,395,805 to Droege et al.;
U.S. Pat. No. 5,081,163 to Pekala; U.S. Pat. No. 5,275,796 to Tillotson;
U.S. Pat. No. 5,086,085 to Pekala; and U.S. Pat. No. 4,997,804 to Pekala,
each of which are hereby incorporated by reference.
A general procedure for the preparation of the aerogel composites according
to the present invention is schematically illustrated in FIG. 4.
Typically, the procedure comprises preparing a nanofiber dispersion
(single individual nanofiber dispersion or nanofiber aggregate dispersion)
in a suitable solvent; preparing a monomer solution; mixing the nanofiber
dispersion with the monomer solution; adding catalyst to the mixture;
polymerizing the monomer to obtain a nanofiber-polymer gel composite and
drying supercritically to obtain a nanofiber-organic polymer matrix
composite. Finally, the nanofiber aerogel composite can be prepared by
carbonizing the aerogel composite.
The nanofiber-polymer aerogel composite can also be prepared by drying the
gel supercritically. If the gel is dried by conventional method (i.e., not
supercritically), a nanofiber-polymer xerogel will be prepared.
Potential applications for the composite aerogels made according to the
invention include those applications for conventional aerogels. The
improvement of mechanical properties resulted from incorporating
nanofibers will make the composite aerogel more attractive and versatile.
Moreover, the increasing in conductivity in the composite aerogels will
result in new applications.
One embodiment of the invention relates to a rigid supported catalyst for
conducting a fluid phase catalytic chemical reaction, processes for
performing a catalytic chemical reaction in fluid phase using the
supported catalyst and a process for making the supported catalyst.
The supported catalyst of the invention comprises a support comprising a
rigid carbon nanofiber structure and a catalytically effective amount of a
catalyst supported thereon.
Rigid supported catalysts of the present invention have unique properties.
They are exceptionally mesoporous and macroporous and they are pure and
they are resistant to attrition, compression and shear and consequently
can be separated from a fluid phase reaction medium over a long service
life. The increased rigidity of the supports of the present invention
enables the structures to be used in fixed bed catalytic reactions. A
packing containing the sized rigid structures can be formed and a fluid or
gas passed through the packing without significantly altering the shape
and porosity of the packing since the rigid structures are hard and resist
compression.
Moreover, the uniquely high macroporosity of carbon nanofiber structures,
the result of their macroscopic morphology, greatly facilitates the
diffusion of reactants and products and the flow of heat into and out of
the supported catalyst. This unique porosity results from a random
entanglement or intertwining of nanofibers that generates an unusually
high internal void volume comprising mainly macropores in a dynamic,
rather than static state. Sustained separability from fluid phase and
lower losses of catalyst as fines also improves process performance and
economics. Other advantages of the nanofiber structures as catalyst
supports include high purity, improved catalyst loading capacity and
chemical resistance to acids and bases.
Rigid structures formed from nanofiber aggregates are particularly
preferred structures for use a catalyst supports. As a catalyst support,
carbon nanofiber aggregates provide superior chemical and physical
properties in porosity, surface area, separability, purity, catalyst
loading capacity, chemical resistance to acids and bases, and attrition
resistance. These features make them useful in packed bed or fluid bed
processes.
Carbon nanofiber catalyst supports have a high internal void volume that
ameliorates the plugging problem encountered in various processes.
Moreover, the preponderance of large pores obviates the problems often
encountered in diffusion or mass transfer limited reactions. The high
porosities ensure significantly increased catalyst life since more
catalyst can be loaded onto the support.
The rigid nanofiber catalyst supports of the invention have improved
physical strength and resist attrition.
The chemical purity of carbon structures has a positive effect on the
selectivity of a supported catalyst since contamination-induced side
reactions are minimized. The carbon structures are essentially pure carbon
with only small amounts of encapsulated catalytic metal compounds
remaining from the process in which the nanofiber was formed. The
encapsulated fiber-forming metal compound does not act as a catalyst
poison or as a selectivity-affecting contaminant.
The combination of properties offered by nanofiber structures is unique. No
known catalyst supports combine such high porosity, high surface area and
high attrition resistance. The combination of properties offered by the
nanofiber structures is advantageous in any catalyst system amenable to
the use of a carbon support. The multiple carbon nanofibers that make up a
carbon nanofiber structure provide a large number of junction points at
which catalyst particles can bond to multiple nanofibers in the nanofiber
structures. This provides a catalyst support that more tenaciously holds
the supported catalyst. Further, nanofiber structures permit high catalyst
loadings per unit weight of nanofiber and this provides a greater reserve
capacity of catalyst. Catalyst loadings are generally greater than 0.01
weight percent and preferably greater than 0.1 weight percent based on the
total weight of the supported catalyst. Catalyst loadings greater than 50
weight percent of active catalyst based on the total weight of the
supported catalyst are easily within the contemplation of the invention,
i.e., loadings in excess of 100 weight percent based on the weight of the
support of the invention, owing to the porosity of nanofiber structures
and other factors discussed herein. Desirable active catalysts are the
platinum group (ruthenium, osmium, rhodium, iridium, palladium and
platinum or a mixture thereof) and, preferably, palladium and platinum or
a mixture thereof.
Because of their high purity, carbon fibril aggregates have the properties
of high purity graphite and, therefore, exhibit high resistance to attack
by acids and bases. This characteristic is advantageous since one path to
regenerating catalysts is regeneration with an acid or a base.
Regeneration processes can be used which employ strong acids or strong
bases. Their high purity also allows them to be used in very corrosive
environments.
The supported catalysts are made by supporting a catalytically effective
amount of a catalyst on the rigid nanofiber structure. The term "on the
nanofiber structure" embraces, without limitation, on, in and within the
structure and on the nanofibers thereof. The aforesaid terms may be used
interchangeably. The catalyst can be incorporated onto the nanofiber or
aggregates before the rigid structure is formed, while the right structure
is forming (i.e., add to the dispersing medium) or after the structure is
formed.
Methods of preparing heterogeneous supported catalysts of the invention
include adsorption, incipient wetness impregnation and precipitation.
Supported catalysts may be prepared by either incorporating the catalyst
onto the aggregate support or by forming it in situ and the catalyst may
be either active before it is placed in the aggregate or activated in
situ.
The catalyst, such as a coordination complex of a catalytic transition
metal, such as palladium, rhodium or platinum, and a ligand, such as a
phosphine, can be adsorbed by slurrying the nanofibers in a solution of
the catalyst or catalyst precursor for an appropriate time for the desired
loading.
These and other methods may be used forming the catalyst supports. A more
detailed description of suitable methods for making catalyst supports
using nanofiber structures is set forth in U.S. application Ser. No.
07/320,564 by Moy et al. entitled "Catalyst Supports, Methods of Making
the Same And Methods of Using the Same", filed Oct. 11, 1994, hereby
incorporated by reference. In U.S. application Ser. No. 07/320,564,
methods of forming catalyst supports with non-rigid nanofiber aggregates
are disclosed. These methods of making and using are suitable for
application in making and using catalyst supports using the rigid porous
nanofiber structures.
Methods of Using Supported Catalysts
Carbon nanofiber structures are candidates for use as catalyst supports for
catalysts that heretofore utilized carbon as a support material. These
catalysts may catalyze substitution--nucleophilic, electrophilic or free
radical; addition--nucleophilic, electrophilic, free radical or
simultaneous; .beta.-elimination; rearrangement--nucleophilic,
electrophilic or free radical; oxidation; or reduction reactions. The
foregoing reactions are defined in March, J. Advanced Organic Chemistry
(3rd ed., 1985) at pp. 180-182. See also Grant and Hackh's Chemical
Dictionary (5th ed. 1987). More particularly, carbon structures of the
invention may be used as catalyst supports for catalysts for slurried
liquid phase precious metal hydrogenation or dehydrogenation catalysis,
Fischer-Tropsch catalysis, ammonia synthesis catalysis,
hydrodesulfurization or hydrodenitrogenation catalysis, the catalytic
oxidation of methanol to formaldehyde, and nanofiber- and/or nanofiber
aggregate-forming catalysts. Typical heterogeneous catalytic reactions and
the catalysts that are candidates for support on rigid porous carbon
nanofiber structures are set forth in Table II below.
__________________________________________________________________________
Reaction Catalyst
__________________________________________________________________________
Hydrogenation
Cyclopropane + H.sub.2 .fwdarw. C.sub.3 H.sub.8
Pt, Pd, Rh, Ru
C.sub.2 H.sub.6 + H.sub.2 .fwdarw. 2CH.sub.4
3H.sub.2 + N.sub.2 .fwdarw. 2NH.sub.3
Fe
2H.sub.2 + CO .fwdarw. CH.sub.3 OH
Cu.sup.+ /ZnO
Heptane .fwdarw. toluene + 4H.sub.2
Pt
Acetone + H.sub.2 .fwdarw. 2-propanol
Pt, Copper chromite
H.sub.2 + aldehyde .fwdarw. alcohol
Pt, Pd, Rh, Ru
nitrobenzene .fwdarw. aniline Pd
ammonium nitrate .fwdarw. hydroxylamine
Pd
alkene .fwdarw. alkane Pd, Pt, Rh, Ru
substituted alkene .fwdarw. substituted alkane
Dehydrogenation Pt
cyclohexanone .fwdarw. phenol + H.sub.2
Aromatization
##STR1## Pd, Pt, Rh
##STR2## Pt
Polymerization Cr.sup.2+ /SiO.sub.2
C.sub.2 H.sub.4 .fwdarw. linear polyethylene
Olefin metathesis Mo.sup.4+ /Al.sub.2 O.sub.3
2C.sub.3 H.sub.6 .fwdarw. C.sub.2 H.sub.4 + CH.sub.3 CH.dbd.CHCH.sub.3
Oxidation
CH.sub.3 OH + 1/2O.sub.2 .fwdarw. CH.sub.2 O + H.sub.2 O
Fe.sub.2 O.sub.3.MoO.sub.3
H.sub.2 O + CO .fwdarw. H.sub.2 + CO.sub.2
Fe.sub.3 O.sub.4, Ni,
CuO/ZnO
1/2O.sub.2 + CH.sub.2 CH.sub.2 .fwdarw. CH.sub.3 CHO
PdCl and similar
salts of noble metals
RCH.sub.2 OH .fwdarw. RCHO + H.sub.2
Pt
Glucose .fwdarw. d-glucuronic acid
Pt
Oligomerization Pd
dimethylacetylene dicarboxylate .fwdarw. hexamethyl mellitate
Isomerization Pd
##STR3##
Carboxylation Rh
CO + CH.sub.3 OH .fwdarw. CH.sub.3 COOH
Decarboxylation Pd
##STR4##
Hydrosilation Pt
SiH(CH.sub.3).sub.3 + cyclooctadiene-1,3 .fwdarw. 3-trimethylsilyl-cyclooc
tene
__________________________________________________________________________
The process of performing a heterogeneous catalytic chemical reaction in
fluid phase with supported catalysts of the invention comprises contacting
a reactant with a supported catalyst in fluid phase under suitable
reaction conditions. The process may be a batch process or a continuous
process, such as a plug flow process or a gradientless process, e.g., a
fluidized bed process. The supported catalysts of the invention are
particularly useful in catalytic processes where the reaction environment
subjects the supported catalyst to mechanical stresses such as those using
liquid phase slurry reactors, trickle bed reactors or fluidized bed
reactors. The attrition resistance and high loading capability of the
supported catalyst are particularly beneficial in these environments.
In a batch process, the reactant(s) are reacted in the presence of the
supported catalyst in a reaction vessel, preferably under agitation, and
then the supported catalyst is separated from the reactant(s)/product(s)
mixture by suitable means for reuse, such as by a filter or a centrifuge.
In a plug flow process, the reactant(s) pass through a stationary bed of
supported catalyst, such that the concentration of product(s) increases as
the reactant(s) pass through the catalyst bed. Any supported catalyst that
becomes entrained in this flow can be separated by suitable means from the
reactant(s)/product(s) stream and recycled into the bed.
In a moving bed or fluidized bed process, the supported catalyst is
fluidized or entrained with the flow of reactant(s) in the process. The
supported catalyst flows concurrently with the reactant(s)/product(s). At
the end of the reaction step, any entrained supported catalyst is
separated from the unreacted reactant(s)/product(s) stream, such as by
filter, centrifuge or cyclone separator, and recycled to the beginning of
the reaction step.
In a fluidized bed process, a bed of the supported catalyst is fluidized
but remains within the bounds of a fixed zone as the reactant(s) move
through the bed and react to form product(s). In this situation any
supported catalyst that becomes entrained in the reactant(s)/product(s)
stream may be separated by suitable means and returned to the fluidized
bed.
In a further form of continuous process, the supported catalyst moves
counter-current to the flow of reactant(s). For example, the reactant may
be introduced as a gas into the base of a vertical reaction vessel and
removed from the top as product(s). The supported catalyst is introduced
at the top of the vessel and cascades turbulently downwardly through the
upward gas flow to be withdrawn from the bottom for recycle to the top of
the vessel. Any supported catalyst entrained in the gas flow exiting the
vessel could be separated and recycled to the top of the vessel for
recycle into the reaction vessel.
The supports of the invention can also be used as supports for what would
otherwise be homogeneous catalysis, a technique sometimes called supported
liquid phase catalysis. Their use as supports permits homogeneous
catalytic processes to be run using heterogeneous catalysis techniques. In
supported liquid phase catalysis, the reactant(s) and catalyst are
molecularly dispersed in the liquid phase that is supported within the
structure of the nanofiber aggregate.
The high internal volume of nanofiber structures, as evidenced by their
porosity, permits them to be loaded with a liquid phase catalyst, much
like a sponge, and used as a catalyst, but in a solid particle form. Each
catalyst-loaded nanofiber structure can be viewed as a microreactor in
that the interior of the structure is loaded with a continuous liquid
phase containing catalyst or a plurality of droplets of catalyst in
solution. Consequently, the structure behaves both as a solid particle for
material handling purposes and as a homogeneous liquid catalyst for
reaction purposes. The usefulness of carbon nanofiber structures is aided
in this regard by their chemical stability. The advantages in using
homogeneous catalyst-loaded nanofiber structures are the ease of
separating the catalyst from the product stream, ease in carrying out the
process, equipment sizing and in avoiding corrosion in the condensed
liquid phase.
Carbon nanofiber structures are amenable to use as supports in the
catalysis of substitutions, additions, .beta.-eliminations,
rearrangements, oxidations and reductions. More specifically, they are
useful in hydroformylation and carboxylation reactions and the Wacker
process.
In carboxylation reactions, a catalyst-loaded carbon nanofiber structure is
prepared by absorbing a solution of the carboxylation catalyst, such as
rhodium chloride and triphenyl phosphine, in a higher boiling point
solvent, such as mesitylene or pseudocumene, into dry carbon nanofiber
structures, such as bird nest carbon nanofiber structures.
The carboxylation reaction is carried out by contacting a vapor phase
feedstock with the catalyst at appropriate temperatures and pressures. The
feedstock mixture may be, e.g., carbon monoxide, methyl acetate, methyl
iodide and solvent. The feedstock is absorbed and molecularly dispersed in
the catalyst solution and reacts in the liquid phase. The reaction can be
carried out in a slurry phase reaction as previously described or in a
fixed bed reaction.
The products of reaction, such as acetic anhydride and/or acetic acid and
byproducts are removed from the fibril aggregate particles by vaporization
or filtration.
In the Wacker Process, a catalyst-loaded carbon nanofiber structure is
prepared by absorbing a catalyst, such as palladium chloride, copper
chloride, potassium chloride or lithium chloride, in a solvent such as
water, into dry carbon nanofiber structures. The loaded catalyst is then
placed into a slurry phase or fixed bed reactor and vapor phase reactants,
such as ethylene, oxygen and hydrogen chloride, are passed through the bed
at appropriate partial pressures and temperatures. The products, such as
acetaldehyde and water can be separated from the catalyst by vaporization
or filtration.
EXAMPLES
The invention is further described in the following examples. The examples
are illustrative of some of the products and methods of making the same
falling within the scope of the present invention. They are, of course,
not to be considered in any way limitative of the invention. Numerous
changes and modification can be made with respect to the invention.
Example 1 (Comparative)
Preparation of a Nonrigid Porous Fibril Mat
A dilute dispersion of fibrils were used to prepare porous mats or sheets.
A suspension of fibrils was prepared containing 0.5% fibrils in water
using a Waring Blender. After subsequent dilution to 0.1%, the fibrils
were further dispersed with a probe type sonifier. The dispersion was then
vacuum filtered to form a mat, which was then oven dried.
The mat had a thickness of about 0.20 mm and a density of about 0.20 gm/cc
corresponding to a pore volume fraction of 0.90. The electrical
resistivity in the plane of the mat was about 0.02 ohm/cm. The resistivity
in the direction perpendicular to the mat was about 1.0 ohm/cm. The mat
was flexible, compressible and easily pulled apart.
Example 2 (Comparative)
Preparation of a Nonrigid Porous Fibril Mat
A suspension of fibrils is prepared containing 0.5% fibrils in ethanol
using a Waring Blendor. After subsequent dilution to 0.1%, the fibrils are
further dispersed with a probe type sonifier. The ethanol is then allowed
to evaporate and a mat is formed. The mat has the same mechanical
properties and characteristics as the mat prepared in EXAMPLE 1.
Example 3 (Comparative)
Preparation of a Low-Density Nonrigid Porous Fibril Plug
Supercritical fluid removal from a well dispersed-fibril paste is used to
prepare low density shapes. 50 cc of a 0.5% dispersion in n-pentane is
charged to a pressure vessel of slightly larger capacity which is equipped
with a needle valve to enable slow release of pressure. After the vessel
is heated above the critical temperature of pentane (Tc=196.6.degree.),
the needle valve is cracked open slightly to bleed the supercritical
pentane over a period of about an hour.
The resultant solid plug of fibrils, which has the shape of the vessel
interior, has a density of 0.005 g/cc, corresponding to a pore volume
fraction of 0.998. The resistivity is isotropic and about 20 ohm/cm. The
resulting structure had poor mechanical properties including low strength
and high compressibility.
Example 4
Preparation of a Rigid Structure from Oxidized Nanofibers
A sample was made from oxidized fibrils which were formed into 1/8"
extrudates and pyrolized to remove oxygen. The density and porosity (water
absorption) was determined to be 0.8 g/cc and 0.75 cc/g, respectively.
The sample was analyzed by Quantachrome Corp. for surface area, pore size
distribution and crush strength. Quantachrome measured a surface area of
429 m.sup.2 /g. The total porosity was measured by N.sub.2
adsorption/desorption. The value determined was 0.83 cc/g (FIG. 2). FIG. 2
shows a substantial absence of micropores, (i.e., <2 nm). The crush
strength for an 1/8 inch extrudate was 23 lb/in.sup.2.
Example 5
Preparation of a Rigid Structure from "as is" Nanofibers
A sample was made from "as is" nanotube CC aggregates (i.e., not surface
oxidized) using phenolic resin/Polyethylene Glycol/Glycerine to hold the
aggregates together. The partially dried slurry was pressed and cut into
.about.1/4" pellets, and pyrolized to remove PEG/Glyderine and convert the
phenolic resin to carbon. The measured density was .0..63 g/cc; water
absorption was 1..0. cc/g.
The sample was analyzed by Quantachrome Corp. for surface area, pore size
distribution and crush strength. The results from Quantachrome indicated a
surface area of 351 m.sup.2 /g. The total pore volume (N.sub.2
adsorption/desorption) was 1.1 cc/g (FIG. 3). The pore size distribution
showed an absence of micropores (less than 2 nm). The crush strength from
a 1/4 inch diameter pellet was about 70 lb/in.sup.2. According to the SEM,
this structure is not homogeneous; it consists of a fairly uniform
distribution of aggregates with fairly large spacings between aggregates,
and smaller spacings between nanotubes in the aggregates.
Example 6
Preparation of a Rigid Structure Using a Gluing Agent
Pellets (1/4") of a composite of polyurethane containing 20 wt % BN fibrils
was pyrolized at 400-800.degree. C. in flowing argon for 6 hrs to remove
all volatiles. Weight loss was 70%. The resulting hard particles were
reduced in volume by .about.33% and had a bulk density of .about.1.0. The
particles were ground in a mortar and pestle without crumbling and sieved
to 100-20 mesh. Internal void volume of the articles was measured by
absorption of water at r.t. to incipient wetness and found to be 0.9 cc/g.
Assuming a true density of 2 g/cc, this corresponds to a void volume of
60%.
Example 7
Preparation of a Rigid Structure Using a Gluing Agent
The procedure in Example 6 was used with a composite of 15 wt % CC fibrils
in polystyrene. Weight loss was 74%. Bulk density was 0.62. Water
absorption at r.t. was 1.1 cc/g, corresponding to an internal void volume
of 69%.
Example 8
Preparation of a Rigid Structure Using a Gluing Agent
A sample of 5.0 g of Hyperion Grade CC Graphite Fibrils.TM. was slurried
for 5 minutes in a Waring Blendor with a cocktail containing 10.0 g
polyethylene glycol 600, 4.7 g phenol, 6.5 g of 35% aqueous formaldehyde
and 500 cc DI water. A thick, stable suspension was obtained which did not
settle after 3 hr. The slurry was transferred to a baffled r.b. flask and
the pH was adjusted to 8.5 with ammonium hydroxide and stirred at
65.degree. C. for several hrs.
The slurry was vacuum filtered in a 2" filter to a thick, pasty filter cake
(2".times.1.5") containing .about.7% fibrils. The cake was further vacuum
dried at 125.degree. C. to a fibril content of .about.15 wt %. At this
point the fibril slurry, still containing residual PEG, glycerol,
phenol-formaldehyde polymer and water could be formed into extrudates,
pellets, or cut into any desired shape. These forms were then vacuum dried
further at 180.degree. C.; there was a 10-15% shrinkage in volume, but no
cracking or breaking of the forms. The formed pieces were then pyrolized
in flowing argon at 650.degree. C. for 4 hrs. Final densities were 0.15
g/cc. Internal void volumes were 6.0 cc/g, corresponding to a 93% void
volume. The formed pieces were much more rigid than untreated fibril
aggregate filter cakes after pyrolysis and could be handled without
breaking. The wet particles could also be handled without breaking, and
removing water by vacuum drying at 120.degree. C. did not weaken the
particles.
Example 9
Preparation of a Rigid Structure Using a Gluing Agent
A sample of 5.0 g Hyperion Grade BN Graphite Fibrils.TM. was treated as in
Example 8. Final density of the formed pieces was 0.30 g/cc; water
absorption was 2.8 cc/g, corresponding to a void volume of 86%.
Example 10
Preparation of a Rigid Structure Using a Gluing Agent
A sample of 5.0 g of Grade CC fibrils was treated as in Example 8, except
that the mixture also contained 5.0 g glycerin in addition to the other
ingredients. Final densities of the formed pieces was 0.50 g/cc. Water
absorption was 2.6 cc/g, corresponding to a void volume of 85%.
Example 11
Preparation of Rigid Structure Using a Gluing Agent
A sample of 5.0 g Grade BN fibrils was treated as in Example 10. Final
densities of the formed pieces was 0.50 g/cc. Water absorption was 1.5
cc/g, corresponding to a void volume of 77%.
Example 12
Preparation of Rigid Structure with Oxidized Nanofibers
A sample of Grade CC fibrils was oxidized with 30% H.sub.2 O.sub.2 at
60.degree. C. to result in a mixed O-functionality on the fibril surfaces.
Carboxylic acid concentrations were determined to be 0.28 meq/g. A sample
of 5.0 g of this material in 500 cc DI water was slurried in a Waring
Blendor with 0.2 g of Polyethyleneimine Cellulose (from Sigma Chemical)
with a base content of 1.1 meq/g. The stable dispersion appeared to be
homogeneous and did not settle after several hours.
The dispersion was filtered and dried to a level of .about.30% fibril
content. The filter cake could be shaped and formed at that point. The
formed pieces were dried and pyrolyzed at 650.degree. C. Densities were
0.33 g/cc. Water absorptions were 2.5 cc/g, corresponding to a void volume
of 85%.
Example 13
Packed Bed Containing Rigid Porous Structures
A 1/2" S/S tube was packed to a height of 6" with 1/8" extrudates from
Example 10. Using a pressure head of .about.10-12" water, water flowed
through the bed at .about.15-20 cc/min without impediment to flow and
without breaking or abrading the particles.
Example 14
Method of Using Rigid Porous Structures in a Fixed-Bed Reactor
A sample of rigid, porous fibril aggregates in the form of .about.1/8"
extrudates as prepared in Example 10 is used to prepare a Pd on carbon
catalyst for use in fixed-bed operation. The extrudates (5.0 g) are washed
in DI water and soaked for 1 hr in 6 N HNO.sub.3. A solution containing
0.5 g PdCl.sub.2 in 6 N HCL is added to the extrudate slurry and the
mixture is stirred in a rotary bath for several hours. The extrudate
particles are separated by filtration and dried at 150.degree. C. and used
in a 0.5" S/S fixed bed reactor to hydrogenate nitrobenzene to aniline.
Example 15
Method of Using Rigid Porous Structures in a S/S Reactor
Extrudates prepared according to Example 11 are used to prepare a
molybdenum on carbon catalyst according to the procedure reported by
Duchet, et al (ref. Duchet, et al, J. Catal. 80 (1983), 386). The catalyst
is loaded into a 1/2" S/S reactor, pre-sulfided at 350.degree. C. in
H.sub.2 S/H.sub.2 and then used to hydrotreat a vacuum oil stream at
350.degree. C. and 0.1 MPa in H.sub.2 to remove sulfur prior to subsequent
further refining.
Example 16
Pressed Disks from Rigid Porous Structures
Samples of Hyperion Grades BN and CC fibrils were surface functionalized by
reaction with 60% nitric acid for 4 hrs at reflux temperature. Carboxylic
acid concentrations were 0.8-1.2 meq/g. After removal of excess acid, the
treated fibrils were partially dried by vacuum filtration and then fully
dried in a vacuum oven at 180.degree. C. at full vacuum. The dried fibril
aggregates were very hard; they could not be cut and had to be ground to
form into shapes. Samples were pressed into 1/8" thick disks at 10,000 psi
in a Carver press using a 1/2" die. Densities of the uncalcined disks
(green) ranged from 1.33 to 1.74 g/cc.
The disks were calcined at 600 and 900.degree. C. to remove surface oxygen.
Densities of the disks were lowered to 0.95 to 1.59 g/cc without weakening
the disks.
Example 17
Testing Mechanical Integrity of Rigid Porous Structures
The rigid particles formed in the Examples were tested for brittleness and
hardness by dropping them (.about.1/4" particles, either as pellets,
extrudates or broken disks) down a 6' tube onto a hard metal surface. The
particles were examined closely for breakage or abrasion. The results are
shown in Table III, along with a summary of the properties of the
materials which were prepared in the examples.
TABLE III
______________________________________
Summary of Physical Properties of Formed Structures
Water
Example Fibril Density Absorp.
Relative
No. Type g/cc cc/g Hardness (1)
______________________________________
6 PU-BN (20%) 0.7 0.9 N
7 PS-CC (15%) 0.6 1.1 N
8 CC (2) 0.15 6.0 B,A
9 BN (2) 0.30 2.8 N
10 CC (2) 0.31 2.6 N
11 BN (2) 0.50 1.5 N
12 CC (3) 0.55 2.5 N
16 BN (Green Disc)
1.74 -- N
16 BN (600.degree. C. Disc)
1.59 -- N
16 BN (900.degree. C. Disc)
1.56 -- N
16 CC (Green Disc)
1.33 -- N
16 CC (600.degree. C. Disc)
1.02 0.50 N
16 CC (900.degree. C. Disc)
0.95 0.50 N
20 0.47 1.75
21 0.45 1.70
______________________________________
(1) N--No Breakage or Abrasion; B = Breakage; A = Abraded
Example 18
Activated Fibril-Aerogel Composites
The preparation of an aerogel composite comprising carbon fibrils was
exemplified by using resorcinol-formaldehyde system.
Materials:
Resorcinol (Aldrich, used as received)
Formaldehyde (37% in H.sub.2 O, Aldrich)
0.2 M Na.sub.2 CO.sub.3
Oxidized Hyperion CC fibrils (5.8%
slurry)
Three samples with different fibril contents (Table IV) were prepared. For
every sample, the resorcinol was first dissolved in H.sub.2 O. After
formaldehyde was added, the solution was mixed thoroughly with fibril
slurry by ultrasonication.
TABLE IV
______________________________________
Starting composition of the samples
Sample No. 1 2 3
______________________________________
Resorcinol 0.333 g 0.333 g 0.333 g
Formaldehyde 0.491 0.491 0.491
Fibril slurry 1.724 3.448 8.879
0.2M Na.sub.2 CO.sub.3
2.6 cc 5.3 cc 7.4 cc
H.sub.2 O 2.6 5.3 5.3
______________________________________
After the addition of Na.sub.2 CO.sub.3 catalyst, the mixture was
transferred to a glass vial. The sealed vial was placed in an oven at
80.degree. C. to polymerize monomers and subsequently crosslink the
polymer. After four days, the samples was removed from the oven. A firm
gel with smooth surface was formed for all three samples. The gel was
washed with water to remove the catalyst. The water in the gel was
exchanged with acetone.
The distribution of fibrils in the polymer matrix was characterized using
SEM. The sample for SEM was prepared by drying Sample 3 in air at room
temperature. The fibrils were dispersed in the polymer matrix uniformly.
Example 19
Fibril-Aerogel Composites
4 g resorcinol was dissolved in 25 cc H.sub.2 O, then 5.898 g formaldehyde
(37% solution) was added to the solution. After adding 0.5 g Hyperion CC
fibril to the solution, the mixture was ultrasonicated to highly disperse
fibrils in the solution. After adding 0.663 g Na.sub.2 CO.sub.3 in 5 cc
H.sub.2 O to the slurry, the slurry was further sonicated to have a
uniform mixture. The gelation was carried out following the procedure
described in Example 18.
The above description of the invention is intended to be illustrative and
not limiting. Various changes or modifications in the embodiments
described may occur to those skilled in the art. These can be made without
departing from the spirit or scope of the invention.
Example 20
A Bakelite Phenolic Resin, BKUA-2370, available from Georgia-Pacific
Resins, Inc., Decatur, Ga., was used as gluing agent for making rigid,
porous extrudates from Hyperion Graphite Fibrils.TM., Grade CC. BKUA-2370
is a heat-reactive phenolic resin dispersed in water/butyl cellosolve at
46 wgt % solids content and is dispersible in water at all dilutions.
A cocktail containing 80.0 g Resin BKUA-2370, 10 g glycerin and 80 g
PolyEthylene Glycol, 600 MW dispersed in water (total volume, 500 cc) was
prepared. It was thoroughly mixed for 10 minutes in a Red Devil mixer.
Five grams of CC Fibrils was treated with 30 cc of the resin cocktail
using a Banbury kneader to obtain a thick, uniform paste. Fibril content
in the resulting slurry was .about.13 wgt %. The slurry was packed into a
50 cc air-driven grease gun, being careful to avoid any air pockets. The
grease gun was fitted with a 3 mm nozzle.
The entire slurry was extruded at 40 psi. The extrudate (uncut) was dried
at 140.degree. C. for 4 hours in air to remove mainly water and partially
cure the resin. The temperature was then slowly increased to 300.degree.
C. for 4 hrs to slowly remove any remaining butylcellosolve, PEG and
complete the curing of the resin. Finally, the extrudates were broken
randomly and calcined in Argon at 650.degree. C. to carbonize the resin.
Recovery weight was 5.3 g. Extrudate diameters were .about.2-3 mm.
The extrudates as produced were slightly hydrophobic. Water droplets beaded
on the particles and only slowly absorbed into the body. However, dilute
acid solutions, e.g., 6N HNO.sub.3, rapidly absorbed into the particles.
After washing the extrudates exhaustively to remove excess acid (pH of
effluent >4) and drying at 120.degree. C., the extrudates were penetrated
rapidly by pure water.
The water absorption capacity (porosity) was determined by saturating a
weighed sample of dry extrudates with water, shaking the extrudate
particles to remove any water adhering, and reweighing. The increase in
weight in grams represents the amount of water absorbed into the particles
in milliliters. These same saturated extrudates were then put into a
measured volume of water. The increase in volume was used as the volume of
the extrudate bodies, and the densities were calculated from the original
dry weights and the increase in volume. The results gave a density of 0.47
g/cc and a water porosity of 1.75 cc/g.
Example 21
Another Bakelite Resin, BKS-2600, a heat-reactive resin solution (54 wgt %)
in ethanol also available from Georgia-Pacific was used to prepare
extrudates from Grade BN Fibrils. A cocktail (500 cc) containing 80 g of
BKS-2600 and 80 g PEG (600 MW) dissolved in ethanol was prepared. A 25 cc
aliqout was used to treat 5.0 g BN fibrils in the same manner as Ex. 20.
Fibril content after kneading was .about.16%.
The slurry was extruded in the same manner as above and dried at
100.degree. C. for 2 hrs to remove ethanol and any other light volatiles,
followed by heating at 140.degree. C. to cure the resin. Temperature was
increased slowly as in Ex. 20 to 300.degree. C. to remove volatiles and
totally cure the resin. Final calcination was done in Argon at 650.degree.
C. Final recovery was 5.2 g.
The extrudates were treated as in Ex. 20 with dilute acid. Water capacity
was 1.70 cc/g, density was 0.45 g/cc.
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